Cached files to many

We are using redhat 2.1 enterprise with a progress database
when using more the 70 users the system cache is growing more then 80 percent of the
total memory and the system going in swap .
result speed and performance is going to 0

After changing the memory from 2Gb to 4 GB the problem is now at 100 users .
or after a time laps 1 day the same problem with 70 users .

Are there setting to change the system cache
Linux is often taking to many memory for cache and not releasing it when needed .

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